USB External Hard Drive Problem

When using a USB external hard drive on my old computer with Windows XP, I could delete and add videos with no problems, and then simply defragment that drive from time to time. Now with my new computer and Windows 7, I am finding that when using the same USB external hard drive to delete and add videos, I am having problems. I remove unwanted files by "moving" them to a folder on my new computer and then when I attempt to copy a new video from my computer, it won't transfer even though there is plenty of storage space on the USB external hard drive. I have found that if I run the error-checking program on that external hard drive, then I am able to transfer files, but that seems very time consuming. Anyone have a solution as to why the external hard drive is acting like it is full when actually there is plenty of space??? The external drive is formatted as NTFS.

Thanks,
Flicka

Yes, I did run a chkdsk to see whether there are any bad sectors.

The security on this disk is set for ALL USERS to have permission to read, write, modify, full control, read & execute.

Should I be "deleting" files I no longer want on this disk instead of "moving" them to a folder on the main computer? Is that causing a problem with not freeing up previously used space on the disk?