Just a tip. I installed 10.10 on a separate partition and I dual boot. I didn't like the fact that my Mavericks partition was getting mounted and things like Spotlight and Time Machine need to be modified in preferences to exclude it. Also its interesting that some items on the old Mavericks HD end up in your application search path.
So the thing to do is make sure it never gets mounted at boot time. The old school sure fire fix is /etc/fstab. Its nice to know Yosemite still uses this file.
This is my /etc/fstab excluding "Macintosh HD"
#
# Warning - this file should only be modified with vifs(8)
#
# Failure to do so is unsupported and may be destructive.
#
LABEL=Macintosh\040HD none hfs rw,noauto
This is the volume structure
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 464.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
4: Apple_CoreStorage 34.3 GB disk0s4
5: Apple_Boot Recovery HD 650.0 MB disk0s5
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS OS X Yosemite *33.8 GB disk1
Logical Volume on disk0s4
3D9797AB-E422-43E7-A1A6-0FE208661A32
Unencrypted
... Jeff
So the thing to do is make sure it never gets mounted at boot time. The old school sure fire fix is /etc/fstab. Its nice to know Yosemite still uses this file.
This is my /etc/fstab excluding "Macintosh HD"
#
# Warning - this file should only be modified with vifs(8)
#
# Failure to do so is unsupported and may be destructive.
#
LABEL=Macintosh\040HD none hfs rw,noauto
This is the volume structure
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 464.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
4: Apple_CoreStorage 34.3 GB disk0s4
5: Apple_Boot Recovery HD 650.0 MB disk0s5
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS OS X Yosemite *33.8 GB disk1
Logical Volume on disk0s4
3D9797AB-E422-43E7-A1A6-0FE208661A32
Unencrypted
... Jeff