External HD won't free-up space after deleting files

Hello. My old Maxtor 500 Gb external hard-drive won't free-up any space after deleting many, many files. I'm using a late 2009 iMac i5 desktop with Snow Leopard. I had 6 Time Machine backups of my old MacBook Pro on the drive - each taking up roughly 70-80 Gb of space. I deleted 3 of the Time Machine backups as they were close enough in date to other backups so I wasn't going to lose any data. This should have freed up more than 200 Gb of space on the external hard drive. However, the hard drive still shows having only 10 Gb of space available. I deleted the Trash from the Dock. The Trash is empty. I unmounted the hard drive, turned it off and reconnected it. I restarted the computer. Still no change. Please help me. Thanks!