time machine 3TB, OS X Mavericks imac, how to delete some backups (need the space for specific data backup) ?

Trash – Can’t Empty or Move File To Trash

Trash FAQ

Time Machine has some complex access control permissions on files & folders. You are going about removing items in a risky way. The Finder has a habit of not wanting to allow you to remove items from a Time Machine backup.

You should probably use the built in method to remove items.

e.g. enter Time machine, find the items & use the gear to remove them.

http://pondini.org/TM/12.html

It's not a good idea to use a Time Machine disk for other backups in my opinion –

• One copy of any important file is a bad idea, two copies on the same disk is equally bad. Entire disks can die, get another disk for some actual redundancy, use it for 'archives' e.g. files that don't need altering anymore.
• On occasion Time Machine has broken for me, so was simply a good idea to erase the backup disk & start over, you can't do that when you need other data from it before erasing.
• Juggling items on the TM disk encourages this type of deleting, eventually you forget how to do it properly, go back to Finder & break things accidentally by dragging to the trash…
• TM will save more history with more space, you may find the file you want has been deleted by TM if backup space gets limited.

Never delete TM files like you would ordinary files. I agree with Drew Reece.

Use disk utility to erase the drive (OSX extended journaled) and format to one partition. In the options tab (at the bottom) be sure it is set to GUID. Then when down open TM preference pane and pick that drive as your TM backup.

If you want anything else on that drive besides TM, I'd suggest you reformat it into 2 partitions - one for TM and a smaller (?) one for other backup things. As TM will slowly take over all the space of whatever size drive it's backing up to, it's best to have it on its own partition.

Also, you should not have your only backups on only one drive; all hard drives will fail at some point. I have my backups backed up so I have two identical copies on different hard drives.

I don't use Time Machine, so i do not know if the app itself needs to be "involved" here, but the normal steps would be to go to Disk utility, highlight that external, and then click on erase. You might need to choose to repartition and don't forget to backup any other data somewhere else temporarily or you might lose everything.

Edit: Disregard my reference to Disk utility - see this document here on how to erase it:

https://support.apple.com/kb/PH5084?viewlocale=en_US&locale=en_US