i erased an external hardisk, connected via USB, first all files, then additional empty space.
Will there be any type of timestamp left for forensic tools to figure out when the disk has been erased?
Possibly. Hard disk space is erased by overwriting it with files containing random data, then deleting those files and overwriting the unused entries in the MFT. File recovery tools can recover the files created by Eraser, and, depending on the circumstances, may be able to recover the original date stamps as well. If the disk in question contains the Eraser Task List, that too will often have a date/time when a task was run.
While Eraser tries to avoid adding to this problem through the use of lower-level system calls, the metadata is scattered all over the drive and it is not possible to fully prevent this problem from happening. I currently lack documentation for the NTFS file system so it is rather difficult to help solve this problem.