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Could be a variety of reasons.<br />
<br />
One simple action to take, which might help, is to run Disk Utility, choosing the repair permissions option.<br />
<br />
This provides a good starting point. Might fix the problem or surface the reason for the problem.
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Retired computer technician

Richard Christensen

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Thanks to the member's who provided answers to my question. &nbsp;What the user of this PC was doing<br />
was unplugging the external backup drive and then plugging it in later. &nbsp;This activity provided considerable<br />
confusion for the Time Machine software. &nbsp;The drive should have been properly ejected electronically.<br />
thanks,<br />
capreol
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I have been using a 4 TB external hard drive to do time machine backups. &nbsp;time machine stopped working<br />
about 6 months ago and it says that time machine is not working...the drive is read only. &nbsp;I have previous time<br />
machine backups on that drive. &nbsp;What has caused this and what should I be doing about it?<br />
Thanks,<br />
capreol
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I have been using a 4 TB external hard drive to do time machine backups.  time machine stopped working
about 6 months ago and it says that time machine is not working...the drive is read only.  I have previous time
machine backups on that drive.  What has caused this and what should I be doing about it?
Thanks,
capreol

Time Machine not working...it says the external drive is read only?

The term "Backup" means the methods and processes involved to copy computer data (system data as well as application data) to media other than the ones where the data originally live (disk, tape, optical, cloud). "Restore" in turn means the methods and processes involved in data recovery, i. e., bringing back copied computer data to their original location. Backup/Restore primarily serves as a means of protection against data loss, be it due to disaster, corruption or sabotage. It can also be used for recovering data from an earlier point in time and even for cloning machines or applications. There is a wide variety of backup/restore software available, from expensive commercial products to free or open source tools.

Storage Software

Storage devices include any device used for storing and retrieving digital information. Hard disk drives (HDDs) use one or more rigid ("hard") rapidly rotating disks (platters) coated with magnetic material. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially. The primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs), but HDDs remain the dominant medium for secondary storage due to advantages in price per bit and per-device recording capacity. CD-RW (Compact Disc-ReWritable) is a digital optical disc storage format that allows information to be stored outside the mechanical HDD.

Storage Hardware

Mac OS X is the desktop operating system from Apple Inc., found on Macintosh computers. OS X consists of a Mach/BSD-based kernel, operating system interfaces primarily based on FreeBSD and additional frameworks (written in C, C++ and Objective-C providing user interface and application-level services.