The problem is that while the drive was originally formatted as HFS+, it was most likely NOT set up with the GUID Partition Table, which is required by Intel Macs if you want the disk to be a boot volume. Yours was probably originally set up with the older APT (Apple Partition Table) which was the standard for PowerPC Macs.
An Intel Mac can read and write files to an APT - HFS+ hard drive, but it cannot boot from one.
When you reformatted the drive with Disk Utility, it used GUID by default.
I know that Disk Utility can change one to the other, but I'm not sure if it can do so without reformatting the disk first. So your reformat did the job, but now you know exactly why.