GRUB installs automatically on sda

When I installed Trisquel 6.0 from a USB device (recognized as sda) on a PATA hard disk (recognized as sdb), the GRUB boot loader was installed to "sda" automatically. I was never asked to which device it should install to.

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[!] Install the GRUB boot loader on a hard disk

Install the GRUB boot loader to the master boot record? Yes

[new screen]
Running "grub-install" on /dev/sda
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This is critical because I could not use my USB device later which could result in permanent loss of personal data. However, this issue was not encountered on a SSD hard disk (recognized as sda) that I bought to avoid this problem.