External USB hard disks only occassionally detected after I instaled a new internal hard disk

Thank you but first I have only one external usb HDD connected at a time and second this happened right after I installed the new internal HDD. I would like to wait for others to give me more options to try. I believe this must have to do something with bios or so.

it will only take you few minutes to run the file.

I have spent a lot of time on this on my machine and I was also surprised that's what it came down to.

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but just in case - try to put the drives in the USB ports at the back of the machine . Sometimes the Front USB panel doesn't get connected properly and it plays up.

First of all you need to narrow the issue to exact description. When you say - it is not detected - do you go to Windows Disk Management and see if you have there this external drive without drive letter assigned?

ok, here are the facts:

the situation is weird, because when I do not work with the external hdd, it is detected, I see it in My computer and in disk managment. As soon as I start coping the data on it, id disconnects - showing me this yellow triangle in the down right corner of monitor with some statement about error in hardware or network and data lost etc.

when ext hdd is not detected:

1. I cannot see it in My computer neither in disk manager
2. I cannot (re)start pc with having external usb hdd connected - it stops in the moment od memory check and does not continue even after esc. Restart immediately resumes only after I disconnect the ext. hdd from pc.

How it happens:

I connect ext usb hdd to pc, and everything is fine = it is detected, it can be seen in My computer and disk managment. Then I start copiyng data on it and after a few seconds it gives me error (see above). Then after me not touching it it starts connecting. And over and over.

It does with all my ext hdd, all my ext hdd are ok in my other two comuters (ps with xp and notebook with vista).

just be aware that this happened after installation of a new internal hard disk WD RE4 1000 GB on OS XP PC - as a replacement (upgrade) of old internal hdd.

I have 2 external hdd, one with power cable and one without it. Both have this problem.

still try it with USB hub.

I have seen some external hard drives that do not handle "bad sectors" properly.  Typically one of two bad things happens if they don't; either
   the drive will "ding-dong, dong-ding" go offline, then back online, such that whatever operation you were in the middle of fails at that point and the drive does autodetect/autorun all over again,
  or, the drive will hang (thus the suggestion to restart the drive by its power, not just the PC)  Many times when a drive goes into error the indicator light on the front of the drive changes colour or flashes an error pattern, but some don't.
basically all usb drives communicate using SCSI commands as per the usb standard, but the drives themselves are usually IDE or SATA, requiring the enclosure to handle translating via a little circuitboard, and frankly some of the enclosure circuit firmware are not well engineered and buggy.

And noxcho is right, trying to power the drives from usb is an iffy proposition, since the usb standard provides barely enough power per bus for one 2.5" drive, and not enough power for 3.5" drives, nevermind anything else over and above.  If you strain the load on your USB bus channel (usually all the usb ports share one single chip/bus, only a handful of units bother to have two usb chip/bus/channels for half the ports each (only usb 3.0 have they increased power by 50%, which is useless wheneverusing it as usb 2 btw)  Powering the drives with AC adaptors and powered usb hubs helps.  Since you've added a second internal drive, the drain of power could be affecting you.

It could also be the disk drivers installed for the new addition have somehow affected drivers for the other drives, it "shouldn't" but of course we've all seen in Windows driver for appliance a adversely affect appliance b, it happens.  This is where how good of an engineering of the devices and their firmware and drivers comes into question.  You might investigate whether there are updated version of the drivers (and if that doesn't work, maybe even firmware updates) for the devices in question.  And of course it could be an issue with the motherboard BIOS, which has to coordinate all of it, internal drives, USB controllers et al., so check that your BIOS is up-to-date and you've the corresponding up-to-date drivers for mobo, chipset, usb from the motherboard maker.

Trying a powered usb seems an easier first step now eh?  In a desktop you could even try adding an additional USB expansion card in hopes it will behave better, often giving you usb 3.0 capabilities and 3.5x faster disk transfer in a system that only had usb 2.

sorry I meant to say "If you strain the load on your USB bus channel "... you can burn-out your usb chip/bus and then all those ports won't power properly or maybe not work forevermore, so be aware.

check your event log to see if there are errors recorded at around the time the thing locks up.  disk errors would suggest everything is fine until you try to do the copy and everytime you try to do the copy you're hitting a bad spot on the disk, probably the external one.  To rectify that you may have to remove the drive from its enclosure so you camn attach it directly to a motherboard hd controller so you can use a program to "fix" the bad spots. (typically you can't do it via usb, because of the scsi command translation circuitboard middle-man abstraction layer that exists between actual drive and computer)  Such programs include Testdisk http://www.cgsecurity.org/wiki/TestDisk  (highly recommend:) HDDRegenerator http://www.dposoft.net/ or SpinRite 6.0 http://www.grc.com/sr/spinrite.htm
Caveat: some external drives act like a network server or can have their own raid mirroring/array internally, such disk appliances require special handling, using special software or built-in routines for repairing the internal raid array etc)

Jezz, guys fist of I want to thank you for paying attention to my topic.

Well when you mentioned this power insuficiency I went back to my WD book external usb with power adapter and  took a notice that this one is fine so it might be my two other 2.5" ext usb hdd without power doing this mess.

So now I am thinking if to buy powered usb hub or give you a credit for the correct answer rightaway. In other words need to decide if I will need my other 2.5" hdds or not but probably yes.

Would this one do?
http://www.alza.cz/EN/maxxtro-gembird-usb2-0-d44639.htm 

One more thing - could this happen after I upgraded internal hdd (have only one in my pc)?
I know this new one consumes more W ... Because I do not remember having this problem with 2.5" hdd before upgrade.

Yes it's possible there is more load on the power supply affecting the feed to usb now that an internal drive is also connected.  In a desktop could try alternate power plug as power supplies usually have a couple of different ganged outputs, but in a laptop there'd be no such option and even more likely to be strained for power.

you still didn't try to remove old USB drivers ?

I told you this long time ago )))))

Does your external HDD have the power cable? Did you try to connect it via powered USB hub?
If not then try it. As you added new drive it could be that your PSU gives not enough power now for all the connections.