What Version of Timeslips? If it is a bku file just rename to a zip and unzip. If newer version give more information about environment. Are you trying to restore onto a network drive or a local drive? What operating system are you using? What is backup stored on? 

Thanks for the reply. The version is N8.0 [04]. I attempted your suggestion of renaming the .bku file to a zip file, but both 7-zip and WinZip say it's not an archive file and can't open it. This is running on Windows 10 32-bit. We also tried restoring any of the backups on a machine freshly installed with XP and that also failed with the same error message regarding disk space. There are backups both on the local drive and on a network drive. 

If you are running v8 then I don’t have much to offer except a recommendation to upgrade. V8 is very old and won’t work on windows 10. You will need a staged upgrade and there’s no guarantees but I’d be happy to discuss and work with you. 

I understand. Thanks for your suggestions. This firm has been running this version on Windows 10 32 bit since Sep 2018. In fact, there was a hard drive failure in early 2019 and the TimeSlips restore operation worked fine in that case. I'm thinking perhaps the difference here is the database has grown significantly larger and so now we are hitting some sort of limitation of the outdated software when trying to restore. I will continue to attempt to extract the slp file from the bku file or otherwise retrieve the data for them if possible. Whether or not successful, I will suggest they upgrade to more modern billing software. Thanks again for your willingness to help.

I understand. Thanks for your suggestions. This firm has been running this version on Windows 10 32 bit since Sep 2018. In fact, there was a hard drive failure in early 2019 and the TimeSlips restore operation worked fine in that case. I'm thinking perhaps the difference here is the database has grown significantly larger and so now we are hitting some sort of limitation of the outdated software when trying to restore. I will continue to attempt to extract the slp file from the bku file or otherwise retrieve the data for them if possible. Whether or not successful, I will suggest they upgrade to more modern billing software. Thanks again for your willingness to help.

If they want to upgrade they should do it soon. Timeslips 2020 is the last perpetual version that Sage will offer. It’s only available until September and must be purchased through a consultant. After September only the premium subscription version will be available. With subscription if you don’t renew each year your database becomes read only. With v8 they are living in borrowed time. You may have outgrown the capacity of version 8 but you are lucky to have it last this long. If you would like pricing please reach out to me.