Error copying files to other drives: "Could not copy file "XXXXXX.XXX" because it's being used."

Since I upgraded my iMac i5 21.5" from OSX Lion to Mountain Lion, I've being facing problems by copying files from my HD to another drives. When I try to copy one or more files to my Time Capsule, or Flash Drive, or SD Card (for example) I frequently receive the message "Could not copy "xxxxxxx.xxx" file because it's being used." after insist some times, the file is transfered. This error is being more and more frequent each time. Curiously, this only happens when copying from my HD to another drive. Never had that copying to the same HD. I have other Macs runing M.L. and S.L. which are doing fine. The only machine that's giving this kind or error is this one. Any tip?