you installed your boot loader on the hdd2 and your computer bios are booting from hdd1 so press what ever F* number button to chose what device to boot from. or set your bios to hdd2 for default boot order.

Thanks all, i think the best way is to install slack on my portable hdd in my home pc as sda and after that everywhere i go i will put in the pc as first drive.
Is there any difference when the linux is loading from my hdd plugged in sata port or with usb interface? I mean to work inside the system not the boot? Thanks

If you're going to be moving the drive around, I'd highly recommend switching to persistent naming (UUIDs) to save yourself the headache when the drive ends up not being /dev/sda. Since each computer is different, some assign devices differently than you'd expect. I can almost guarantee it will happen at some point. You'll end up with a kernel panic and be unable to boot the system without some rescue media. Using persistent naming in your lilo.conf and fstab will completely prevent that from occurring. All you'd need to do is select your drive in the boot menu on any computer and you'll be golden :)

As far as the difference, you'd probably only notice the speed difference, since USB tends to not have the same speeds as SATA (unless you're on USB3). Otherwise, everything should work as normal, with the only possible exception being that your /dev/sd* devices might not match what you expect. But you can always reference them by /dev/disk/by-*/. The various subfolders under there should match whatever hardware you have it hooked up to, and they're a symlink to whatever the device ended up being called (so your UUID won't change, but it will always point to the correct /dev/sd* entry).