I have an external hard drive with Photo on it. When I tried to open the application it says "There is not enough disk space to open your library." The external Hard Drive is 1 TB and it says I've used 1 TB. If I can't open the photos, how do I free up space?
Mac Studio, macOS 15.6
In addition to Richard's questions - How large is the Photos Library you are trying to open?
When you keep a Photos Library on an external drive, you do not only need the enough storage to hold the library, but you need plenty of free storage as a working space for Photos, when it needs to repair or rebuild the library. Be prepared for the worst case, that the library size can temporarily increase by fifty percent. When I tried to open an old Photos Library I have saved on a Mac with an older system version, the size of the library increased temporarily by 50%. And if you are using iCloud Photos the size of the library can also grow considerably, if Photos needs to download additional originals from iCloud.
If the library has indeed a size of 1TB, copy it to a drive with at least 2TB of free storage, so Photos will have enough free working storage. For Photos Libraries that you are keeping as an archive of older photos it is much more convenient to split them into smaller libraries. It will make upgrading, rebuilding, repairing much easier, because it will be faster for a small library and we do not need to provide so much free storage. And it is safer not to put all eggs into the same basket - if a library gets damaged, the other may still be safe.
In addition to Richard's questions - How large is the Photos Library you are trying to open?
When you keep a Photos Library on an external drive, you do not only need the enough storage to hold the library, but you need plenty of free storage as a working space for Photos, when it needs to repair or rebuild the library. Be prepared for the worst case, that the library size can temporarily increase by fifty percent. When I tried to open an old Photos Library I have saved on a Mac with an older system version, the size of the library increased temporarily by 50%. And if you are using iCloud Photos the size of the library can also grow considerably, if Photos needs to download additional originals from iCloud.
If the library has indeed a size of 1TB, copy it to a drive with at least 2TB of free storage, so Photos will have enough free working storage. For Photos Libraries that you are keeping as an archive of older photos it is much more convenient to split them into smaller libraries. It will make upgrading, rebuilding, repairing much easier, because it will be faster for a small library and we do not need to provide so much free storage. And it is safer not to put all eggs into the same basket - if a library gets damaged, the other may still be safe.
Photos is an app that stays in the Applications folder. The data is in the Photos Library which is normally in the Pictures folder, but it can be moved to an external drive-- is that what is on your external drive? When you open Finder's Info Window, does it actually say 1 TB?
Does the drive have only Photos Library on it?
And how is your external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:
Move your Photos library to save space on your Mac - Apple Support
If this drive is in a an incompatible format, stop running Photos with it immediately! A Photos Library can sit on an incompatible drive, but running it may corrupt the database.
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There is not enough disk space to open your library.
