Time Machine - accidentally moved all backup files to Trash, need to put them back to ext. hard drive but cannot

I have an external hard drive for Time Machine to back up my iMac (under 10.6.8). Last night, I accidentally put all back-up files from TM drive to Trash (instead of selecting a single folder of a week worth of back-up files, I selected the iMac's back-up folder itself from the folder Backups.backupdb). Oops. I attempted to put the iMac back-up folder (containing several weeks worth of back-up files) back into the folder Backups and I was prompted to "authenticate" then a message telling me: The item "(name of my iMac)" can't be moved because "Backups.backupdb" can't be modified. I clicked on Authenticate, then saw a message: "The operation can't be completed because backup items can't be modified." I ciicked ok and nothing happen.

I searched online for advice how to fix this but most of the search results have more to do with how to recover deleted files from Time Machine drives or whatever. Nothing about how to put or restore back-up files back into a TM hard drive from Trash. It's about 370 GB of back-up files and I don't have enough space in my primary hard drive for it. I tried to eject the TM drive and unplugged the USB but it did not resolve it.

Please help ASAP. Thank you.