tar -cf filename.tar path/to/small/files/*
to pack, tar -xf filename.tar
to unpack.
tar
is vailable for example via MinGW, or UnxUtils or GnuWin32. Peazip and 7z can create tar files too, but they are pretty pathetic performance-wise compared to the "genuine" tool. On my computer, the genuine tar
utility runs 5-6 times faster on large numbers of small files (no idea why, it's just copying data from one file to another!).
Since your files are already ZIP files, it is unlikely that compression will further reduce the size. On the other hand, compression is usually in the low range of megabytes per second whereas disk read is in the hundreds. Thus, compression would considerably increase the time you spend creating that archive, and simply using tar
is probably best.
Corruption should not be that much of an issue since typical transports (say, FTP) are reliable, and the underlying protocols and network layers are pretty good at checksumming and detecting bit errors.
Still, you might consider creating several smaller tar-files because if you only transfer one huge file and the FTP server at the other end crashes (or your internet connection gets a hiccup) after you have transferred 1.49 of your 1.5 TB, this will be pretty annoying. With somewhat smaller files, you don't need to resend that much.