OS Mirrored hard drives getting full

OS is on a pair of mirrored 145GB drives   <-- So you have 145 GB of space. Not enough anymore. 300 GB outright minimum and think 500 GB as a better approach and even more if you wish to store lots of data.

I would suggest backing up the drives (Windows Server Backup would be appropriate), remove the two drives, install two new drives, set them up as an array, then boot the Server 2008 DVD and restore the backup.

It is pretty straightforward and "safe" given that you'll not be changing anything on the original pair of drives.

I think Lee W has/had reference on what can be cleared from C: drive to make available room
C:\windows
Look for long ago updates, and delete these folders. That shoukd free things up,
Look at temp space, user space to see whether there are downloads that could be consolidated elsewhere.
The other option is to replace the 146Gb drives one at a time with a 300Gb one until the raid1 is optimal, at which point you could expand using diskpart...... Make sure you have a good backup before trying ........

If a similar server exists for testing purposes.
...

Arnold, so, if I understand correctly, I can replace one of the 145GB drives with, say, a 500GB drive and let the mirror rebuild. Once that is done I would replace the other drive and let the mirror rebuild again. Then use a partition manager to expand the array to use the extra space?

I've already scrubbed the drives of everything I can think of to gain some space... temp files, updates, etc... I gained some breathing room but not enough to last forever. User files are already on a separate RAID5 array.

What consumes space, what roles are installed on the server's c:\
Wsus? You can move the content to the raid5 wsusutil potentially freeing up 20-40Gb depending on your settings.

Not sure you can go to 500, presumably your raid1 are sas drives 146GB, you would have to use the same interface drives 300,450,600,900 are the sas drive capacities..

What is your upgrade cycle, it might be replaced with newer....system in the near future before the issue cones up again.

Check your page give management to make sure it is not the space consumer through automated page space management on c:\ drive......

This is a single server for a small company. All data is on the separate RAID5 array so the only thing on the mirrored array is the OS to include Active Directory, DHCP and DNS, This is Standard Edition Server 2008R2. Nothing special. I'll check the page file mgmt. User directories are on the Data array.

Based on your defined roles, 146Gb shoukd be enough.
I do not see a component that would consume so much storage.

you can take 1 drive and image it to the 500 GB drive  - then remove both 145 Gb drives, and install 2x 500
now reboot and let it rebuild
this way you keep both old drives intact, if something goes wrong
you can use the free paragon soft for imaging - or any other one
http://www.paragon-software.com/home/br-free/download.html

OK. They are SAS drives so I now see that there is not a 500GB option but I'm sure 450 will do. I will try to run disk cleanup tonight. Thank you both. I'll let you know how it turns out.

I am not sure this is possible since I have not tested it but will through the idea out there.

What is you removed one of the drives in the mirror and added a large one 300 , 500GB etc.. Let the drives sync and do the same of the other drive. I am wondering if Windows would see the extra space as available and be able to expand the drive??

Long shot.. Also did you use spacesniffer and check for any dmp files?

Windows will only see the available space after the expanding of the RAID volume on the RAID controller. only after the volume is expanded on the RAID controller will windows see the extra space and will need to use diskpart to expand/resize the C:\ partition...

I would concur with arnold.  The problem is that not all controllers allow expanding the volume.  If we know that your controller allows this, then it is a good process.  If it does not, the best you'll get with that method is a second volume at the end of the array, which I assume is not what you want.

Ran CleanMgr and moved Page Files to the RAID5 array. Picked up 36 GB. Still need to think about up-sizing hard drives in the near future but maybe can hold out long enough to just replace the server this time around.

Your system runs SQL Server or Exchange?
Set a 2GB limit on the page file on C: I would not set such a large file on the RAID 5...
How many drives are in use out of the available?
Potentially instead of using another RAID1 volume as a drive letter, it could be set as a path....
When pressed there are ways, but I think the freeing/moving of things has extended the duration.
If you can add memory to the system it might reduce the need for the page file.... while improving performance of the system.

NO SQL or Exchange. I checked out the server today and there are actually some empty drive slots. I had thought there weren't. This should make it easy to image the boot disk and restore the image to a new, larger drive, boot to it then restore the mirror to the larger drives, yes? Arnold, when you say "Potentially instead of using another RAID1 volume as a drive letter, it could be set as a path...." please explain this a little more. I'm not certain what you are referring to. Thank you all for all of your help thus far.

Look at disk management interface, you have the option to use the created using a drive letter, the other option deal with using the partition in a psth similar to how UNIX/Linux work
Let's say you have a data folder c:\somepath\data
This space consumes a large amount of space that could be better managed.
You create a new raid load it as a drive letter E:\, copy the data, rename c:\sonepath \data to data2
Go back to disk management and now attach this drive instead of  using a drive letter E: you would attach it as c:\somepath\data
Permissions gave to be checked since .....

Once you confirm all is good, the space in data2 can be reclaimed for c:\.

I see.