IBM RT User's Setup Manual page 168

Fixed-Disk Drive (Type R70). If a third fixed-disk drive is not
installed, slot 2 can be used for a different adapter.
If the IBM RT PC Extended ESDI Magnetic Media Adapter is
installed in adapter slot 1 of an IBM 6150, up to three IBM RT
PC Extended ESDI Fixed-Disk Drives (Type E) and two diskette
drives can beinstalled internally. In this case, a second drive
adapter is not needed, and a different adapter can be used in
slot 2.
Warning:
Use of the IBM RT PC Extended ESDI
Magnetic Media Adapter requires use of the IBM RT PC
Advanced Processor Card (APC) or the IBM RT PC
Enhanced Advanced Processor Card, together with IBM RT
PC Extended ESDI Fixed-Disk Drives (Type E). The IBM
RT PC 70MB Extended ESDI Fixed-Disk Drive (Type E70)
and the IBM RT PC 70MB ESDI Fixed-Disk Drive (Type
R70) have different formats and are
Either an IBM Personal Computer AT Fixed-Disk and Diskette
Drive Adapter, an IBM RT PC ESDI Magnetic Media Adapter,
or an IBM RT PC Extended ESDI Magnetic Media Adapter can
be installed in slot 6 of an IBM 6151; additional internal drives
cannot be added to this system.
IBM RT PC Personal Computer AT Coprocessor Option
If you have an IBM RT PC Personal Computer AT Coprocessor
Option, it must be installed in adapter slot 8 of the IBM 6150 or
in slot 5 of the IBM 6151. If you do not have the IBM RT PC
Personal Computer AT Coprocessor Option, you can install a
different adapter (but
Option) in these two slots.
IBM PC Enhanced Graphics Adapter
The IBM PC Enhanced Graphics Adapter must be installed in
either slot 3 or 6 of the IBM 6150 and in slot 1 of the IBM 6151.
interchangeable.
not
an AT 512 KB Memory Expansion
not
Section 1. Getting Started 1-17